With miniaturization and densification of electronic devices, the use of flip-chip mounting technology via bumps has been active.
At present, the following methods are carried out: a method wherein bumps on a copper base are connected by soldering (Patent Literature 1, Non-patent Literature 1), a method wherein a palladium or gold thin film is formed on a copper and/or nickel base, and connected by adding solder (Patent Literature 2, Non-patent Literature 2), an electrolytic plating method (Patent Literatures 3 and 4), a dry method, an electroless plating method and others.
In the method wherein bumps on a copper base are connected by soldering, lead-free solders are actively used, leading to easy occurrence of problems in the reliability of connections. Furthermore, whiskers with a length of several μm to several hundreds μm are generated, frequently causing problems, and this is disadvantageous in reducing pitch size. In addition, electroless soldering has a problem in that thick soldering is difficult.
In the method wherein a palladium or gold thin film is formed on a copper and/or nickel base and connected by adding solder, while nickel having high hardness can be used as a mother material of bumps, its direct use in the connection of solid layers as in the case of gold is difficult, and it is not suitable for microsize with a width of 100 μm or less. In this method as well, soldering is used, and there is a problem of generation of whiskers.
In the case of a nickel mother material, a thin gold film is formed on said mother material, then fusion joining by soldering is carried out; in this case also, the above problems occur.
An electrolytic plating method is sometimes adopted as a method for producing bumps made of gold alone; however, this method has a problem in that it cannot form a film on a closed pattern.
In addition, when a plurality of microfine structures such as gold bumps are produced by an electrolytic plating method, structures with different heights tend to be generated, and as the pattern becomes finer, it becomes more difficult to form a plurality of bumps with a uniform height, i.e., a plurality of microfine structures with a uniform size.
For example, this problem of non-uniformity can be eliminated by smoothing of bumps using special polishing (chemical mechanical polishing (CMP)) after the plating process (Non-patent Literature 3). Cu is used in such processing, and its chemical dissolution is easy.
However, the Au structure of the present invention is chemically more stable than Cu, and therefore this method cannot be applied directly.
Technically, bumps made of gold alone can be produced by a dry method. When microfine structures such as gold bumps are produced using the dry method, there are problems such as that a large amount of the targeting material is consumed, and a long processing time is necessary; therefore they have not yet been practically adopted.
Meanwhile, in the case of an electroless plating method using electroless gold plating that does not accelerate deposition on fine sections, microfine structures with a desired height can be obtained by long-term processing or an increase in the deposition rate. However, since a resist is designed on the premise of patterning and removal, it is generally weak to high temperature and high-alkaline conditions; specifically, when a resist is immersed in an aqueous solution with a temperature of 60° C. or more for one hour or longer, cracking, swelling and elution of components tend to occur easily. In addition, even when the temperature is less than 60° C., the resist may sometimes be damaged by immersion for several hours.
Furthermore, when the gold deposition rate is increased regardless of the pattern size, bathes tend to show self-decomposition, so that practical application is difficult.
In the case of a cyanogen-containing electroless gold plating solution, conditions can be set such that a film-forming rate of 5 μm or more per hour is achieved even on a planar section, in addition to fine sections, while a self-decomposition reaction hardly occurs. However, cyanogen-containing high-speed electroless gold plating solutions comprise cyanogens that have an aggressive nature to various materials including metals, and also they are highly alkaline of pH 9 or more; thus damage to the resists can hardly be avoided.
Furthermore, gold plating used in the electroless plating method to date is of a substitution-reaction type wherein gold deposition of only less than 0.1 μm is possible, and even for thick plating, only up to 1 μm is possible, and the use of soldering is a premise.
Namely, with the conventional methods, microfine patterns with a size of micrometer order, in particular those with a width of 100 μm or less and a height of 3 μm or more can be hardly formed.